Evidence of Granular Superconductivity in Pr2<-zCezSr2Cu2NbO10

Pr2-zCezSr2Cu2NbO10 (Pr222Nb10), is widely believed to not superconduct, although its homologues in which Pr is replaced by Nd, Sm, Eu, or Gd do (with T-c approximate to 28 K). On the basis of bulk resistivity, bulk diamagnetism, electron spin resonance, and surface resistance measurements of Pr222Nb10, we conclude that it does exhibit granular superconductivity (as predicted) with a critical temperature in the range 25-28 K, but is not currently a bulk superconductor. Neutron diffraction studies show that our Pr222Nb10 is multi-phase, but does not contain significant amounts of any (known) superconducting phase other than Pr222Nb10. It does contain similar to 23% Pr-Sr defects, but far fewer Sr-Pr impurities (<2%), so that the predicted antistructure defect (Sr-Pr,Pr-Sr) is probably not responsible for breaking Cooper pairs and destroying superconductivity, but the important half of it, Pr-Sr, almost certainly is.